I'm not writing as an expert, just as some schmuck with an opinion...
When I'm recording someone else, I spend a few minutes listening, and crawling around. Sometimes, I stick a finger in one ear. After a while, when I've found a spot or two where the sound is really happening (whatever that means for the particular project), then, I stick a mic there. Depending on how patient the talent is, I might try 3-4 mics, moving them around slightly and playing with the angles. If the talent is impatient, I'll go to a standby (MC012), and fidget as little as possible. Often times, I'll try to record a scratch take or two, and play with a couple of choices there.
I try to avoid mic placement via headphones if possible, because my headphones suck. If I had better ones, I might try, but even so, it'll only get you so far.
When I'm recording my self (masturtracking?), I'll take more liberties. Whenever I get a new mic, I put it up on the stand, pick a tune, and record 10-15 takes, to get a sense of how the mic is working with my instrument / voice. That way, when I'm sketching ideas, or recording solo, I can just pick one up, and have a pretty good idea of how it'll perform.
Caveat #1: I haven't spent much time with recording drums
Caveat #2: I haven't spent much time recording groups with anything other than a stereo pair.
Man, sometimes it's like the blind leading the blind...
-mg